Till now, DE2 driver supported only UI planes. Before we add support for
VI planes, lets split out UI layer specific code from common parts. This
commit does the following:
- renames sun8i_layer.c to sun8i_ui_layer.c
- moves UI channel specific code to sun8i_ui_layer.c
- moves common code from sun8i_layer.c to sun8i_mixer.c
- renames function and structure names so it is apparent where they
belong to

No functional change is made.
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171201060550.10392-19-jernej.skrabec@siol.net